1. Field of the Invention
The present invention relates to optical fiber cables. More particularly, the present invention relates to buffer tubes for optical fiber cables which are reinforced with a substantially continuous composite material.
2. Description of the Prior Art
Optical fiber cables are used to transmit information at high rates and very long distances. The transmission medium of the optical fiber cable are hair-thin optical fibers. These fibers are protected from external forces and elements by precisely designed and manufactured cable structures, such as buffer tubes for example.
To protect the hair-thin optical fibers, it is desirable to have a buffer tube made from a material with a high Young's modulus. The use of a material with a high Young's modulus results in a buffer tube having relatively high tensile and compressive resistance capability, a trait useful in protecting the optical fibers in the event a cable incorporating such buffer tubes is twisted, stretched or compressed. In addition to the high Young's modulus, it is also important that the material selected for making a buffer tube have a low thermal expansion coefficient. Significant shrinkage or expansion of the core or buffer tube caused by temperature changes can place undesirable tensile or compressive loads on the optical fibers residing therein. High tensile or compressive loads can result in damaged or broken optical fibers or cause too much microbending which results in significant signal attenuation. Of course, in addition to the physical properties of materials, the cost of raw materials and the cost of processing such materials is also a factor that must be taken into account when designing an optical fiber cable. Finally, the material must have good handling characteristics that enable a technician to gain access to the fibers protected by the tube made therefrom.
In the past, single layer optical fiber cable buffer tubes have been used to protect such optical fibers. These single layer tubes have been manufactured from materials such as polybutylene terephthalate (PBT) or polyamide (PA). PBT buffer tubes were widely used in the past but due to the rigidity, hydrolytic instability and relatively high density and material cost of PBT, cables can now be made with polypropylene buffer tubes reinforced with glass fibers as disclosed in U.S. Pat. No. 5,561,729, or polypropylene/polyethylene copolymer buffer tubes as described in U.S. Pat. No. 5,574,816. As those skilled in the art will appreciate, the physical properties of single layer tube designs are obviously limited by the materials which make up the tube. One drawback of the glass fiber reinforced polypropylene buffer tubes disclosed in U.S. Pat. No. 5,561,729 is that such tubes can be very stiff and therefore make access to the fibers very difficult.
Dual layer buffer tubes made from a layer of a polyolefin material and a layer of a polyester material, such as that described in U.S. Pat. No. 5,031,996, have been proposed to overcome some of the physical property limitations of single layer buffer tubes. However, such dual layer tubes are known to be difficult to process and difficult to handle in the field due to high tube rigidity.
Accordingly, it is desirable to provide a buffer tube structure that has some of the physical property advantages provided by the dual layer construction without the known processing and handling difficulties. It is also desirable to provide a buffer tube made from a material which experiences low post extrusion shrinkage.